1. Field of the Invention
Embodiments of the invention generally relate to the field of semiconductor manufacturing processes and devices, more particular, to methods of controlling defects for extreme ultraviolet lithography (EUVL) photomask substrate.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
With the shrink of critical dimensions (CD), present optical lithography is approaching a technological limit at the 45 nanometer (nm) technology node. Next generation lithography (NGL) is expected to replace the conventional optical lithography method, for example, in the 32 nm technology node and beyond. There are several NGL candidates, such as extreme ultraviolet (EUV) lithography (EUVL), electron projection lithography (EPL), ion projection lithography (IPL), nano-imprint, and X-ray lithography. Among these, EUVL is the most likely successor due to the fact that EUVL has most of the properties of optical lithography, which is more mature technology as compared with other NGL methods.
A photomask is typically a glass or a quartz substrate having a film stack with multiple layers, including a light-absorbing layer, an opaque layer, a capping layer and so on sequentially disposed thereon. As feature sizes formed on the photomask have become smaller, the demand for high quality substrate with minimum defect has increased. Impurities, such as pits, particles, scratches, or other defects, contained in the quartz substrate may adversely cause detectable phase shifts in the beam reflected from the quartz substrate during a lithography process, resulting inaccurate feature size transfer. Furthermore, impurities formed in the substrate may cause small topographic variations on the substrate surface, which may later affect film growth and morphology. Small topographical variations from the substrate surface may also translate to variations to the film stack formed thereon, thereby distorting the structures of the film stack and affecting feature translation to the film stack.
Therefore, there is a need to improve quality of the substrate with minimum defects for EUVL fabrication.